Interpretive Summary: In mature stands of switchgrass, tillers constitute the greatest percentage of harvestable biomass, and their cellular composition will determine both quality and management practices. Although much is known about switchgrass agronomy and breeding, there is limited data on the anatomical and compositional aspects of this important forage and biomass crop. In this work we examined cell anatomy and composition gradients in tillers of switchgrass with the aim of developing baseline information that could guide future breeding and management research. Flowering tillers were collected from plants raised from seeds in a greenhouse, harvested at soil level and separated into internodes beginning with the node subtending the peduncle. Internodes were analyzed using microscopy, by fiber digestion and HPLC. Microscopic observations indicated the development and maturation of thick-walled cortical fibers which eventually occupied a 100-125 µm wide region just beneath the epidermis in the oldest internodes. With time the highly lignified vascular fiber sheath eventually became confluent with the cortical fibers, creating a tissue that could impact quality. Compositional analyses indicated increasing cellulose and lignin contents with increasing age of the internode. There was a simultaneous decrease in cell-solubles and hemicelluloses. A similar trend was observed for both soluble and wall-bound phenolics, which were greatest in amounts and complexity in younger internodes and declined in levels in older internodes. The changes in phenolics were consistent with other data indicating increased lignification of the older internodes. This study provides important insights into tiller development and demarcates tissues that can be targeted for future manipulation. Changes in plant anatomy as well as in composition accomplished through breeding strategies are likely to yield germplasm with superior quality and management traits.

Technical Abstract:
This work examined cell composition gradients in mature tillers of switchgrass with the aim of developing baseline information on this important forage and biomass crop. Flowering tillers were collected from plants raised from seeds in a greenhouse, harvested at soil level and separated into internodes beginning with the node subtending the peduncle. Internodes were analyzed using microscopy, by fiber digestion and HPLC. Scanning electron microscopy and light microscopy demonstrated the development and maturation of cortical sclerenchymatous fibers which eventually occupied a 100-125 µm wide region just beneath the epidermis. In older internodes, the thick-walled vascular fiber sheath eventually became confluent with the cortical fibers. Toluidine blue staining indicated significant lignification of the cortical fibers, vascular fiber sheaths, xylem, and possibly the parenchyma in the most mature internodes. Detergent fiber analysis indicated increasing cellulose and lignin contents with increasing age of the internodes. There was a simultaneous decrease in cell solubles and hemicelluloses. A similar trend was observed for both soluble and wall-bound phenolics, which were greatest in amounts and complexity in younger internodes and declined in levels in older internodes. The changes in phenolics were consistent with other data indicating increased lignification of the older internodes.